This invention relates to the field of thermal processing of cast articles and more particularly to a method of heat treating cast alpha/beta titanium alloys and the articles produced thereby.
The alpha/beta titanium alloys are well known in the art and are described in "Titanium and Titanium Alloys Source Book" published by the American Society for Metals (1982). In particular, the physical metallurgy, properties, microstructure and conventional processing of titanium castings are discussed in this publication in Pages 289-300. The alpha/beta titanium alloys and processes applicable thereto are the subject of U.S. Pat. Nos. 3,007,824, 3,405,016, 3,748,194, 3,901,743, 4,053,330. U.S. Pat. No. 3,007,824 discloses a surface hardening process applicable to a specific alpha/beta alloy which involves heating the article to a temperature within the beta phase field and then quenching. No further heat treatment or modification of the resulting microstructure is utilized. U.S. Pat. No. 3,405,016 describes a heat treatment to improve the formability of alpha/beta titanium alloys which involves quenching from the beta phase field followed by mechanical deformation in the alpha/beta phase field. U.S. Pat. No. 4,053,330 describes a method for improving the fatigue properties of titanium alloy articles which requires deformation in the beta phase field to refine the beta grain size, followed by rapid quenching to a martensitic structure and tempering in the range of 1000.degree. to 1600.degree. F. to partially convert the martensite to acicular alpha and cause the formation of discrete equiaxed beta particles at the acicular alpha boundaries.
Titanium alloys are often used in applications where a high ratio of mechanical properties to weight is important. Specifically, such alloys are typically used in dynamic applications such as fan and compressor blades in gas turbine engines where a high level of tensile and fatigue strengths is critical. However, these strength characteristics of the selected alloy must be accompanied by good toughness, and high resistance to impact damage and crack propagation. The alpha/beta titanium alloys in which the alpha and beta phases are present at low temperatures are commonly used for these applications. In order to use these alloys effectively in such dynamic applications the wrought or forged processing conditions are conventionally utilized because of their superior fatigue strength compared to that of castings produced from the same alloys. Similarly, critical static structural use of titanium castings in gas turbine engines has often been limited by the inferior mechanical properties compared to that of forgings. Nevertheless, the lower cost of titanium castings compared to machined forgings establishes a significant incentive to improve the properties of castings so that they are competitive with those of forgings.
In many gas turbine engine applications the ability to utilize a cast titanium alloy article with an attractive balance of tensile strength, impact and crack propagation characteristics is particularly desirable. Such applications include but are not limited to hollow titanium airfoil shapes such as blades and vanes. In many cases hollow components are necessary to reduce component weight or to improve their functional performance. For example hollow titanium airfoils allow fan stage blades to be designed with high structural stiffness to weight ratios. Hollow titanium fan airfoils make it possible to eliminate the midspan shroud which is often used to eliminate excessive blade vibratory deflection due to aerodynamic loading. Very low aspect ratio airfoils become possible as a result of hollow blade construction which can also result in improved aerodynamic efficiency and improved resistance to impact from ingested foreign objects such as birds.
The construction of such hollow titanium airfoils has been demonstrated by several schemes of manufacture including the welding, brazing or diffusion bonding of multiple pieces to produce a single hollow structure. However, each of these approaches has associated undesirable aspects such as excessive cost, metallurgical inhomogeneity in chemistry or microstructure or difficulty in controlling the presence of sharp internal notches which can lead to premature fatigue failure. A hollow cast titanium airfoil produced by conventional investment casting practice utilizing a leachable internal core minimizes or eliminates these shortcomings when processed according to this invention.
It is the object of this invention to provide a cast titanium fan blade, solid or hollow, having a controlled alpha/beta structure derived from a prior martensitic condition.
It is another object of this invention to provide a cast titanium alloy hollow fan blade having fatigue strength comparable to a wrought fan blade.
It is a further object of this invention to provide a process for transforming the microstructure of a cast titanium alloy into an alpha/beta phase structure derived from a prior martensitic condition.
Cast titanium alloy articles produced from the class of titanium alloys which contain both alpha and beta stabilizer may be heat treated by the method of this invention to improve their fatigue behavior while maintaining high resistance to impact damage and propagation of cracks. The process produces a metallurgical structure of randomly oriented acicular alpha, with no large colonies of similarly aligned alpha platelets, and with control over the width of individual alpha platelets which leads to a very desirable and advantageous balance of fatigue properties with other mechanical properties.
The present invention is practiced by heat treating a cast titanium alloy article at a temperature above its beta transus temperature for a time sufficient to achieve a substantially beta microstructure, and thereafter rapidly cooling the article to produce an acicular martensitic microstructure. The resulting martensite is then thermally decomposed by stabilizing the article at a temperature within the alpha/beta phase field to form acicular alpha and beta phases, and to grow the alpha platelets to a predetermined thickness to provide them with the desired characteristics. Thereafter, the article is cooled to room temperature. The article is then aged by reheating it to a temperature between about 1000.degree. to 1300.degree. F. for a time of about 1 to 8 hours to partially decompose the beta phase, thereby achieving the final desired properties.